1. Field of the Invention
The present invention relates to an electric motor, and, more particularly, to a permanently excited direct current electric motor comprising a housing carrying a magnetic pole and a grounding ring arranged on it.
2. Prior Art
Grounding or short-circuiting rings made from magnetically conducting material, e.g. from steel, are used in permanently excited direct current motors, in order to conduct the magnetic flux of the permanent magnet to the stator. Furthermore, the motor housing itself, or an additional steel ring, may be employed for this purpose.
A motor housing of a small electric motor is known from German Patent Document DE 28 10 21 5 A1, in which a housing carrying a permanent magnetic sector of a stator is made from corrosion-resistant galvanized steel plate. A grounding ring that is similarly made from corrosion-resistant steel plate is arranged on the housing in order to provide a sufficiently large grounding cross-section for conducting the magnetic flux in a motor of greater power.
In another known small motor described in German Patent Document DE 42 24 628 A1 the pole housing carrying the magnetic pole is made from a corrosion-resistant sheet metal, for example from galvanized steel plate, and has a grounding ring made from a similar corrosion-resistant sheet metal material on its outer side. Fins that are folded around the wall of the pole housing without play are cut into the end faces of the cylindrical grounding ring, so that the grounding ring is at least secured axially on the pole housing.
In operation of this sort of motor the vibrational modes of the housing itself or of the grounding ring-housing system may be easily excited, so that the housing and/or the grounding ring-housing system generates sound transmitted by the surrounding air at the eigenfrequencies of the housing or the grounding-ring housing system. Additional damping devices and/or damping measures are usually required to take care of this undesirable situation.